Method for fabricating a top magnetoresistive sensor element having a synthetic pinned layer

ABSTRACT

A method for forming top and bottom spin valve sensors and the sensors so formed, the sensors having a strongly coupled SyAP pinned layer and an ultra-thin antiferromagnetic pinning layer. The two strongly coupled ferromagnetic layers comprising the SyAP pinned layer in the top valve configuration are separated by a Ru spacer layer approximately 3 angstroms thick, while the two layers in the bottom spin valve configuration are separated by a Rh spacer layer approximately 5 angstroms thick. This allows the use of an ultra thin MnPt antiferromagnetic pinning layer of thickness between approximately 80 and approximately 150 angstroms. The sensor structure produced thereby is suitable for high density applications.

RELATED PATENT APPLICATION

This application is related to Ser. No. 09/458,727, filing date Dec. 13,1999 now abandoned, and to related Ser. No. 09/769,813, filing date Jan.26, 2001 now U.S. Pat. No. 6,620,530, both assigned to the same assigneeas the current invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to the fabrication of a giantmagnetoresistive (GMR) magnetic field sensor in a magnetic read head,more specifically to a spin valve type of GMR sensor having a syntheticantiferromagnetically pinned (SyAP) layer.

2. Description of the Related Art

One of the most commonly used structural configurations of magnetic andnon-magnetic layers in giant magnetoresistive (GMR) read-heads is theso-called spin-valve magnetoresistive (SVMR) structure. In the mostbasic version of the SVMR, two ferromagnetic layers such as CoFe or NiFeare separated by a thin layer of electrically conducting butnon-magnetic material such as Cu. One of the layers has itsmagnetization direction fixed in space or “pinned,” by exchange couplingwith an antiferromagnetic (AFM) layer, usually a layer of MnPt, directlydeposited upon it. The remaining ferromagnetic layer, the unpinned orfree layer, can rotate its magnetization vector in response to smallvariations in external magnetic fields such as are produced by movingmagnetic media, (which variations do not affect the magnetizationdirection of the pinned layer). The rotation of one magnetizationrelative to the other produces changes in the resistance(magnetoresistance) of the three layer structure. A constant currentsent through the SVMR then produces voltage variations across it, whichare sensed by external circuitry. More germane to the present inventionare newer versions of the SVMR that utilize a so-called syntheticantiferromagnetically pinned (SyAP) layer which is a tri-layeredlamination comprising two ferromagnetic layers separated by a thinmetallic, non-magnetic layer and wherein the two ferromagnetic layersare magnetically oriented in antiparallel directions by exchangecoupling. In the SVMR configuration, this SyAP pinned layer would thenbe pinned by an additional antiferromagnetic material (AFM) pinninglayer. Methods for fabricating several versions of this SyAP SVMR havebeen taught in the prior art. Gill, (U.S. Pat. No. 6,122,150) teaches aformation in which an SyAP tri-layer is formed of two 20 A layers ofCo₉₀Fe₁₀ separated by an 8 A layer of Ru. This tri-layer is exchangecoupled to an antiferromagnetic pinning layer of 425 A of NiO. AlthoughGill teaches several other refinements of this structure, it is the SyAPtri-layer that exemplifies the prior art for our purposes. Huai et al.(U.S. Pat. No. 6,175,476 B) teaches the formation of a SyAP pinned layerwith high resistivity and improved thermal stability by using a 4-10 ARe (Rhenium) layer rather than a Ru (Ruthenium) layer as theantiferromagnetic coupling layer. Huai also teaches an annealing methodfor setting the domain state of the AFM pinning layer by heating thepinning layer above its blocking temperature and then cooling it in thepresence of an applied magnetic field. The applied magnetic field alignsthe domain state of the adjacent pinned layer which fixes the domainstate of the pinning layer. Pinarbasi (U.S. Pat. No. 6,201,671) teachesthe formation of a bottom SVMR sensor (a configuration in which the AFMpinning layer is vertically below the pinned and free layers) in whichan NiO AFM layer is formed on a TaO seed layer for the purpose ofimproving the SVMR magnetoresistance (dR/R). Pinarbasi (U.S. Pat. No.6,208,491) teaches the formation of a capping structure for a SyAPpinned layer SVMR to improve its magnetoresistance under hightemperature conditions. Finally, Pinarbasi (U.S. Pat. No. 6,208,492 B1)teaches the formation of a bilayer seed structure on which is formed anantiferromagnetic pinning layer for a SyAP pinned layer.

As magnetic storage media densities increase, the shield-to-shieldthickness of the SVMR must correspondingly decrease to provide thenecessary resolution of the rapid magnetic flux changes. To decreasethis thickness, the SVMR stack, including all layers that contribute toits operation, must itself be made thinner. Since the thickest layer inthe SVMR stack is the antiferromagnetic (AFM) pinning layer (e.g. anMnPt layer of thickness exceeding 150 A for a recorded density of 30Gb/in²), it becomes desirable to reduce the thickness of that layer.Another reason that reducing the AFM pinning layer thickness would beadvantageous, is that a portion of the sensing current necessary forsensor operation is shunted through the pinning layer. This current lossreduces the ultimate magnetoresistive sensitivity of SVMR operationbecause the shunted portion of the current is unaffected by resistancechanges and cannot contribute to the voltage variations that areultimately sensed. However, thinning the AFM layer will reduce theexchange bias energy (J_(ex)) between that layer and the pinned layer.In addition, it is found that the AFM pinning layer produces anotherdisadvantageous effect, it creates hysteresis effects (open R-H loops)in the relationship between R (magnetoresistance) and H (externalmagnetic field). This hysteresis is due to the AFM induced anisotropy,H_(ck), which leads to sensor instability. Unfortunately, when the AFMpinning layer is reduced in thickness to improve sensor resolution, theinduced anisotropy is not reduced although the pinning energy is.Therefore, the hysteresis effect becomes worse.

On the other hand, a SVMR sensor for higher recording densities requiresa higher pinning strength so that the smaller and more rapid externalfield variations can be more accurately sensed without hysteresis. Wehave found (and will show below), through simulations and empiricalresults, that AFM pinning layer thickness can, in fact, be reduced ifthe coupling between the two antiparallel ferromagnetic pinned layers ofthe SyAP, AP1 and AP2, can be improved. It is to this end that thepresent invention is directed.

SUMMARY OF THE INVENTION

A first object of this invention is to provide a method for forming aSVMR sensor having a thinner stack and, therefore, decreasedshield-to-shield spacing.

A second object of this invention is to provide a method for forming aSVMR sensor capable of reading magnetic media with storage densities upto and exceeding 70 Gb/in².

A third object of this invention is to provide a method for forming aSVMR sensor having an enhanced GMR ratio.

A fourth object of this invention is to provide a method for forming aSVMR sensor having improved hysteresis properties.

A fifth object of this invention is to provide a method for forming aSVMR sensor having an increased ESD threshold.

A sixth object of this invention is to provide a method for forming aSVMR sensor in a manner that provides a larger annealing window (rangeof annealing temperatures) than is provided by methods taught within theprior art.

A seventh object is to provide the sensor so formed by the methods ofthe present invention.

In accord with the objects of this invention there is provided a methodfor forming an SVMR sensor element having a syntheticantiferromagnetically pinned (SyAP) layer and an antiferromagnetic (AFM)pinning layer wherein the AP1/AP2 coupling between the two ferromagneticlayers is improved by the use of an ultra-thin non-magnetic couplinglayer and whereby the use of an extremely thin antiferromagnetic pinninglayer is thereby permitted. In this context, the AP2 layer is the pinnedferromagnetic layer closest to the AFM layer, whereas the AP1 layer isthe pinned ferromagnetic layer that is closest to the Cu spacer layerthat separates the antiferromagnetically pinned synthetic tri-layer fromthe ferromagnetic free layer. Further in accord with the objects of thepresent invention there is provided a method for forming a SVMR sensorelement in a top spin valve configuration, since the pinning field forsuch a top spin valve configuration is found to be stronger than that ofan equivalent bottom spin valve configuration, even though the industrytrend is towards the formation of bottom spin valves. Further in accordwith the objects of the present invention there is provided a method forforming a synthetic antiferromagnetically pinned (SyAP) layer for such atop spin valve configuration, wherein said SyAP layer is of the formCoFe 20 A/Ru 3 A/CoFe 15 A. Because of the high (>20 kOe) saturationfield, H_(s), of this SyAP layer, usual annealing methods to fix theantiferromagnetic coupling are not possible. To achieve the objects ofthe present invention, therefore, a low field annealing process as setforth in related patent application HT 99-011 and fully incorporatedherein by reference, is applied. Yet further in accord with the objectsof this invention there is provided a method for forming a SVMR sensorof the bottom spin valve type also having the advantageous propertiesdiscussed above. Said bottom spin valve type, however, cannot use a SyAPlayer with a 3 A Ru spacer due to pinhole formations, so a thicker Rhspacer layer of thickness between 4-6 A is formed. Said Rh based SyAPstructure has an even greater H_(s) than the Ru layer structure, so thelow field annealing process discussed above must also be applied. Theadvantageousness of the present method for eliminating soft ESD damageis fully set forth in related patent application HT 00-032, which isincorporated herein in its entirety by reference.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects, features and advantages of the present invention areunderstood within the context of the Description of the PreferredEmbodiments, as set forth below. The Description of the PreferredEmbodiments is understood within the context of the accompanyingfigures, wherein:

FIG. 1 is a graph of exchange coupling energy, J_(s), for the SyAP as afunction of Ru and Rh spacer thickness.

FIG. 2 shows a measured R-H graph of a bottom SyAP SVMR structure:NiCr55 A/MnPt100 A /CoFe15 A/Ru3 A/CoFe20 A/Cu20 A/CoFe 10 A/NiFe20A/Ru5 A/Ta20 A. The poor hysteresis illustrates the fact that the Ru3 Aspacer is too thin.

FIG. 3 is a graph showing the low field annealing window (usableannealing fields to maintain high GMR ratio) for top Ru3 A and bottomRh5 A spin valves.

FIGS. 4a and 4 b are schematic drawings of top and bottom spin valvesensor stacks formed according to the method of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides a method for forming both top and bottomtype spin valve magnetoresistive (SVMR) read sensors having syntheticantiferromagnetically pinned (SyAP) layers (two ferromagnetic layersseparated by a non-magnetic spacer layer and magnetized in mutuallyantiparallel directions) which are exchange coupled to a pinning layerformed of antiferromagnetic material (AFM). The novelty andadvantageousness of the invention is its provision of a method forforming such a SVMR sensor with an exceptionally thin stack, which is aresult of said stack having both a thin AFM pinning layer and anultra-thin non-magnetic spacer layer separating the ferromagnetic layersin the SyAP pinned layer.

Reducing the AFM thickness, while maintaining pinning strength, has beenfound to be possible by improving the coupling between the twoantiparallel (in magnetization) ferromagnetic layers, AP1 and AP2through the use of ultra-thin non-magnetic spacer layers.

From simulation results we conclude that a strong coupling between AP1and AP2 is necessary to allow the AFM to be reduced in thickness. Sinceit is known that using thin (<6 A) layers of non-magnetic Ru, Rh or Irproduces a larger exchange coupling energy than a Ru7.5 A (angstrom)layer, we should expect that a more effective SVMR sensor can be madeusing such thin layers of these materials.

Referring to FIG. 1, therefore, there is shown graphical evidence of theeffect of reducing the thickness of Ru and Rh spacers in the SyAP layer.As can be seen, a Ru spacer of thickness between 3-4 A, or a Rh spacerof thickness between 4-6 A, each provide a higher coupling energy than aRu7.5 A spacer. The highest coupling field produced by Ru is at 3 A,while the highest coupling field produced by Rh is at 5 A.

Based on these results, a top spin valve comprising a SyAP layer of theform: CoFe 20 A/Ru 3 A/CoFe 15 A was fabricated. In manufacturing themore usual SyAP spin valve configuration with a Ru7.5 A spacer (CoFe 20A/Ru 7.5 A/CoFe 15 A) which has a measured saturation field H_(s)=6.0kOe, an annealing field of 10 kOe can be used. Because the saturationfield of the Ru 3 A spin valve is much higher (>20 kOe), the annealingmust be done using another method, the low field annealing process. Thislow field annealing process is set forth completely in related patentapplication HT 99-011, which is incorporated herein in its entirety byreference.

We conclude that the experimental results confirm the simulations andthat with strong coupling through a thin, Ru 3 A spacer layer, the MnPtlayer can be reduced in thickness without sacrificing, and evenimproving, pinned layer integrity and performance.

Recently there has been an industry trend towards using the bottom spinvalve (BSV) configuration. For an equivalent pinned layer structure tothat of the top spin valve (TSV) type, it is found that the pinningfield for the bottom spin valve is much smaller than that of the topspin valve. For example, the pinning fields measured for a TSVconfiguration:

NiCr/NiFe/CoFe/Cu/CoFe20 A/Ru7.5 A/CoFe15 A/MnPt 150 A

and a BSV configuration:

NiCr/MnPt150 A/CoFe15 A/Ru7.5 A/CoFe20 A/Cu/CoFe/NiFe

respectively, are 3200 Oe and 2100 Oe. The pinning fields for SyAPstructures made with a Ru 3 A spacer are, respectively, about 6000 Oeand 2600 Oe. Referring to FIG. 2, there is shown the R-H loop for the Ru3 A SyAP BSV, indicating a very large loop opening. It is believed fromthis result that growing a Ru 3 A spacer layer, which is less than twomono-layers thick, in the BSV configuration, produces a layer withpinholes.

As is indicated in FIG. 2, a Ru 3 A spacer layer is too thin for use ina bottom SyAP spin valve. In the present invention we propose the use ofa Rh 5 A spacer layer for the bottom SyAP spin valve. Since thesaturation field, H_(s), of the Rh 5 A SyAP bottom spin valve structureis even higher than that of the Ru 3 A SyAP top spin valve structure,the bottom spin valve structure must be annealed by the low fieldannealing process of related patent application HT 99-011, incorporatedherein in its entirety by reference.

To verify the conclusions relating to MnPt thicknesses reached above bysimulations, two SyAP configurations were fabricated and tested. Theseconfigurations were:

NiCr/MnPt(x)/CoFe/Ru7.5 A/CoFe/Cu/CoFe/NiFe/Ru/Ta: x=100, 120, 150, 175A

NiCr/MnPt(x)/CoFe/Rh5 A/CoFe/Cu/CoFe/NiFe/Ru/Ta: x=80, 100, 150 A,

where the first Ru7.5 A configuration served as a reference.

To verify the efficacy of low field annealing, SyAP structures:

CoFe15 A/Ru3 A/CoFe20 A (TSV)

CoFe15 A/Rh5 A/CoFe20 A (BSV)

were fabricated and studied.

Referring to FIG. 3, there is shown low field annealing data for the twofabrications. Comparing the Ru to the Rh cases, it can be seen that theRh has a larger annealing window because the Rh structure has a largercoupling energy than the Ru structure. For the Rh bottom spin valves anannealing field of 2000 Oe was used.

Referring finally to FIGS. 4a and 4 b, there is shown a schematicdiagram of the stack layer formation for a top spin valve SyAP (4 a)formed in accordance with the present invention and for a bottom spinvalve SyAP (4 b) formed in accordance with the present invention. FIG.4a shows an NiCr seed layer (2), formed to a thickness of betweenapproximately 45 and 60 A (angstroms), which is found to enhance suchmagnetoresistive properties of the sensor as layer smoothness andthermal stability, over which is formed a first ferromagnetic free layer(8) which is a bilayer comprising an NiFe layer (4) formed to athickness of between approximately 0 A and 50 A over which is formed aCoFe layer (6) of thickness between approximately 5 A and 30 A. Overthis free layer (8) there is then formed a metallic, non-magnetic spacerlayer (10), which in this example is a Cu layer formed to a thickness ofbetween 16 A and 25 A. This spacer layer separates the free layer fromthe pinned layer. Over the Spacer layer is then formed the pinned layer(18), which is a tri-layer comprising first (12) and second (14)ferromagnetic layers, designated AP1 and AP2, separated by a thinmetallic, non-magnetic spacer layer (16). In this example the firstferromagnetic layer, AP1, (12) is a layer of CoFe formed to a thicknessof between approximately 10 A and 25 A and the second ferromagneticlayer, AP2, (14) is a layer of CoFe formed to a thickness of betweenapproximately 10 A and 25 A. The spacer layer (16) is a layer of Ru,which is formed to a thickness of between approximately 3 and 4 A, butis most advantageously formed to a thickness of approximately 3 A. Thisultra thin Ru layer provides a strong coupling between the twoferromagnetic layers allowing them to be given antiparallelmagnetizations and coupled into a layer (18) (a SyAP layer) which willbe antiferromagnetically pinned by the antiferromagnetic layer (20) thatis subsequently formed. The said antiferromagnetic pinning layer (20),which is a layer of MnPt, is then formed on the pinned layer (18) with athickness of between approximately 80 A and 150 A. Note that thedesignation “AP2” always refers to the ferromagnetic layer in closestproximity to the pinning layer. A capping layer (19), which can be alayer of NiCr or NiFeCr between approximately 20 A-30 A thick, may beformed on the antiferromagnetic pinning layer. A low field annealingprocess in a magnetic field of approximately 2,000 Oe completes theformation and antiferromagnetically pins the SyAP tri-layer (18) to theMnPt antiferromagnetic pinning layer (20). The extreme thinness of theMnPt layer (20) is largely responsible for the thinness of the entireformation, which is necessary for a sensor having the resolutionnecessary for reading high density magnetic storage media. As waspointed out in the earlier discussion, it is the strong coupling betweenthe two antiparallel ferromagnetic layers (12)&(14), mediated by theultra-thin Ru layer (16) that allows the formation of such a thinpinning layer.

Referring next to FIG. 4b, there is shown a bottom spin valve formationformed in accordance with the present invention and also displaying theadvantageous thinness that is provided by a strongly coupled SyAP pinnedlayer and a thin MnPt pinning layer. An antiferromagnetic pinning layerof MnPt (22) is first formed on an NiCr seed layer (20), of thicknessbetween approximately 45 and 60 angstroms, which seed layer is found toenhance the magnetoresistive properties of the sensor. The MnPt layer isformed to a thickness of between approximately 80 A and 150 A. On thepinning layer is then formed a pinned layer (28), which is a tri-layercomprising a first (24) and second (23) ferromagnetic layer, denoted AP1and AP2 respectively, separated by a thin metallic, non-magnetic spacerlayer (26). In this example the first ferromagnetic layer, AP1, (24) isa layer of CoFe formed to a thickness of between approximately 10 A and25 A and the second ferromagnetic layer (23) is a layer of CoFe formedto a thickness of between approximately 10 A and 25 A. The spacer layer(26) is a layer of Rh, which is formed to a thickness of betweenapproximately 4 and 6 A, but, for strongest coupling, is mostadvantageously formed to a thickness of approximately 5 A. This ultrathin Rh layer provides a strong coupling between the two ferromagneticlayers allowing them to be given antiparallel magnetizations and becoupled into an SyAP layer (28) that is antiferromagnetically pinned bythe antiferromagnetic layer (22) that is first formed. A metallic,non-magnetic spacer layer (30) is then formed on the AP1 layer (24) ofthe pinned layer (28), said spacer layer in this example being a layerof Cu formed to a thickness of between approximately 16 A and 25 A. Overthe spacer layer is formed a ferromagnetic free layer (36), which can bea bilayer comprising a layer of CoFe (32) formed to a thickness ofbetween approximately 5 A and 30 A and over which is then formed a layerof NiFe (34) formed to a thickness of between approximately 0 A and 50A. A capping layer (38), which can be a layer of Ta or TaO, formed to athickness of between approximately 5 A and 20 A is then formed over thefree layer (36). The fabrication is then annealed in a low strengthmagnetic field of approximately 2,000 Oe, to provide the necessaryantiferromagnetic coupling between the pinned layer and the pinninglayer. As is the case with the top spin valve of FIG. 4a, this bottomspin valve also has the advantageous property of extreme thinnessnecessary for resolving magnetic data stored at high densities.

As is understood by a person skilled in the art, the preferredembodiment of the present invention is illustrative of the presentinvention rather than limiting of the present invention. Revisions andmodifications may be made to methods, materials, structures anddimensions employed in practicing the method of the present invention,while still remaining in accord with the spirit and scope of the presentinvention as defined by the appended claims.

What is claimed is:
 1. A method for fabricating a top spin valvemagnetoresistive (SVMR) sensor element having a synthetic, stronglycoupled antiferromagnetically pinned (SyAP) tri-layer and thinantiferromagnetic pinning layer, suitable for high linear densityapplications, comprising: providing a substrate; forming on thesubstrate a seed layer to enhance giant magnetoresistive properties ofsubsequently formed sensor elements; forming on said seed layer a firstferromagnetic layer, which will be a ferromagnetic free layer; formingon said first ferromagnetic layer a first metallic, non-magnetic spacerlayer; forming on said first spacer layer a second ferromagnetic layer(AP1); forming on said second ferromagnetic layer a second metallic,non-magnetic spacer layer of Ru, formed to a thickness betweenapproximately 3 and 4 angstroms or Rh, formed to a thickness betweenapproximately 4 and 6 angstroms, which is of sufficient thinness toprovide strong antiferromagnetic coupling thereby of said secondferromagnetic layer to a third ferromagnetic layer; forming on saidstrong coupling second non-magnetic spacer layer said thirdferromagnetic layer (AP2), which will be strongly coupled to the secondferromagnetic layer through said second spacer layer, forming, thereby,said synthetic, strongly coupled antiferromagnetically pinned (SyAP)tri-layer having antiparallel magnetizations; forming on said thirdferromagnetic layer (AP2) a thin antiferromagnetic pinning layer ofMnPt, NiMn, IrMn or MnPdPt, formed to a thickness of betweenapproximately 80 angstroms and approximately 150 angstroms; forming onsaid antiferromagnetic pinning layer a capping layer; annealing saidstructure in a low external magnetic field and thereby stronglyantiferromagnetically pinning said antiferromagnetic layer to saidsynthetic, strongly coupled antiferromagnetically pinned (SyAP)tri-layer.
 2. The method of claim 1 wherein the seed layer is a layer ofNiCr or NiFeCr formed to a thickness of between approximately 45angstroms and approximately 65 angstroms.
 3. The method of claim 1wherein the first ferromagnetic layer is a laminated bilayer comprisinga layer of NiFe formed to a thickness of between up to approximately 50angstroms on which is formed a layer of CoFe formed to a thickness ofbetween approximately 5 angstroms and approximately 30 angstroms.
 4. Themethod of claim 1 wherein the first metallic, non-magnetic spacer layeris a layer of metallic, non-magnetic material chosen from the groupconsisting of Cu and CuAg and is formed to a thickness of betweenapproximately 16 angstroms and approximately 25 angstroms.
 5. The methodof claim 1 wherein the second ferromagnetic layer (AP1) is a layer ofthe ferromagnetic material of CoFe or CoFeB.
 6. The method claim 1wherein the second ferromagnetic layer (AP1) is a layer of CoFe and isformed to a thickness of between approximately 10 angstroms andapproximately 25 angstroms.
 7. The method of claim 1 wherein the thirdferromagnetic layer (AP2) is a layer of ferromagnetic material chosenfrom the group consisting of CoFe and CoFeB.
 8. The method of claim 1wherein the third ferromagnetic layer (AP2) is a layer of CoFe formed toa thickness of between approximately 10 angstroms and approximately 25angstroms.
 9. The method of claim 1 wherein the capping layer is a layerof NiCr or NiFeCr formed to a thickness of between approximately 20angstroms and approximately 30 angstroms.
 10. The method of claim 1wherein the second non-metallic spacer layer is Ru and the fabricationis annealed in an external field between approximately 1,750 Oe andapproximately 2,250 Oe.
 11. The method of claim 1 wherein the secondnon-metallic spacer layer is Rh and the fabrication is annealed in anexternal field between approximately 1,000 Oe and approximately 2,500Oe.